The present disclosure relates to a sheet post-processing device. This sheet post-processing device is added to an image forming apparatus such as a copying machine, a printer, and the like.
Priority is claimed on Japanese Patent Application No. 2011-032446, filed Feb. 17, 2011, the content of which is incorporated herein by reference.
A sheet post-processing device is attached to an image forming apparatus. The sheet post-processing device is configured so that a sheet ejected from the image forming apparatus in series is stacked neatly. An image is formed on the sheet. The sheet post-processing device is also configured to perform a sorting process arranging the position of the sheet for each stack. Furthermore, the sheet post-processing device is configured to perform a post-processing operation for each stack. An example of the post-processing operation includes performing a stapling process for each stack.
The sheet post-processing device performs a shifting operation in order to neatly stack a plurality of sheets. The shifting operation refers to an operation of shifting the sheet in a width direction (i.e., a direction perpendicular to the direction in which the sheet is transported). When this shifting operation is performed for each sheet, the duration of time required for the shifting operation becomes too long considering the speed at which a sheet is supplied by the image forming apparatus. As a result, a waiting time occurs at the image forming apparatus side. Hence the productivity of the image forming apparatus declines.
In order to solve these problems, conventionally, a holding mechanism is provided inside the sheet post-processing device which holds a sheet. As a result, sheets are stacked, and a shifting process is performed.
Normally, a sheet post-processing device performs a shifting process for a bundle of only a few sheets such as two sheets or three sheets. As a result, when a stack of sheets includes more than the number of sheets included in one sheet bundle or unit, the shifting process is performed for each sheet bundle. Thus, the shifting process is performed for all of the sheets included in the stack of sheets. Thereafter, the stack of sheets is ejected.
When the number of sheets included in one unit equals two, and a shifting process is performed, the sheet post-processing device temporarily holds the first sheet supplied by the image forming apparatus. Two sheets supplied by the image forming apparatus are stacked. Thereafter, the shifting process is performed.
When the number of sheets included in one unit equals three, and a shifting process is performed, the sheet post-processing device temporarily holds the first and second sheet supplied by the image forming apparatus. The third sheet supplied by the image forming apparatus is stacked with the first and second sheets. Thereafter, the shifting process is performed.
During an arranging process, when the number of sheets in one stack of sheets is not equal to a multiple of the number of sheets in one unit on which the shifting process is performed, an extra sheet remains at the end.
For example, when the number of stacks of sheets ejected is one, a problem does not occur. However, when a plurality of stacks of sheets are ejected, it is necessary to change the position of the stack of sheets that is ejected after the stack of sheets that is ejected first. As a result, the shifting position of the sheets is altered.
Therefore, it is not possible to stack a sheet, included in a stack ejected later, with an extra sheet that remains at the end of a stack ejected earlier. Hence, in order to prevent any waiting time of the image forming apparatus, it is necessary to prevent the extra sheet from being held in the sheet post-processing device. Alternatively, it is necessary to eject the extra sheet after only a short amount of holding time.
However, even when it is attempted to prevent the extra sheet from being held in the sheet post-processing device, or to eject the extra sheet after only a short amount of holding time, a waiting time of the image forming apparatus occurs when the shifting process of the stack ejected earlier is not yet completed because the extra sheet cannot be sent in the forward direction.
The present disclosure is made according to these considerations. An object of the present disclosure is to reduce any waiting time of an image forming apparatus due to an operation of a post-processing device. Another object of the present disclosure is to enhance the productivity of an image forming apparatus.